1. Field of the Invention
The present invention relates to a positioning control apparatus, and more particularly, to an apparatus for performing positioning at high speed with high accuracy.
2. Description of Related Art
FIG. 1 is a block diagram illustrating a positioning control apparatus with a switching manner between speed control and positioning control which has generally been adopted as a head-positioning control apparatus for use in a magnetic disc apparatus. FIG. 1 is a typical example of the positioning control apparatus operating at high speed with high accuracy. The apparatus is described in such references as .circle.1 R. K. Oswald, Design of a Disk Head-Positioning Servo, IBM Journal of Research and Development, vol. 18, No. 6, November 1974, .circle.2 R. K. Oswald, Head Positioning Servo Design For The IBM 3344/3350 Disk Files, IEEE TRANSACTIONS ON MAGNETICS, vol. MAG-14, No. 4, July 1978, .circle.3 Takanami et al., Peripheral Circuits for 3.2 GByte Multi-Device Disk Storage, Transactions On Research and Utility By Nippon Telephone and Telegraph Company, vol. 31, No. 1, 1982.
In the description described hereinafter, a command will be assumed to be referred to as a command signal. In FIG. 1, reference numeral 1 designates an object to be controlled, concretely, a motor and an access head integrated with the motor. A speed signal 4 (a speed of the access head) of the object 1 is detected by a speed detector 3. The speed signal 4, then, is converted to a position signal 6 by an integrator 5, and a position error 7 between the position signal 6 and a position command 2 is obtained. A speed error 10, which is the difference between the speed signal 4 and a speed command 9 obtained with respect to the position error 7 from a speed reference curve 8 (a speed-position curve when the object 1 being accelerated at the maximum acceleration and decelerated at uniform deceleration), is inputted to a compensator 12 through a switch 11. In the compensator 12, frequency of the inputted speed error 10 is amplified and attenuated, and a control input 13 is outputted by the compensator 12 to the object 1. A position signal 15 (a position of the access head) of the object 1 is detected by a position detector 14, and a position error 16 between the position signal 15 and the position command 2 is inputted to the compensator 12 through the switch 11. In the compensator 12, frequency of the inputted position error 16 is amplified and attenuated, and the control input 13 is outputted by the compensator 12 to the object 1.
A positioning control apparatus of the prior art comprises both a speed control system for obtaining output response at high speed and a position control system for performing positioning with high accuracy. Those control systems arranged in parallel to each other can be switched over between the speed control system and the position control system by the switch 11, whereby positioning control is carried out at high speed with high accuracy.
Operations of the both control systems will now be described below. In the speed control system, the speed signal 4 is controlled so as to be in accord with the speed command 9 by such a process as that feedback compensation is applied through the compensator 12 to the speed error 10 between the speed command 9 and the speed signal 4 detected from the object 1 by means of the speed detector 3, and that the control input 13 is generated therein.
In order that a high-speed control is performed, however, there exists a problem of saturation of the control input 13 to the object 1. In order to obtain high-speed response by a control performing only the feedback compensation, it is necessary to momentarily provide an excessive control input. Then, in order to carry out high-speed control within the limits of control input, the apparatus of the prior art has employed a quadratic curve called a speed reference curve as shown in FIG. 2 so as to obtain the speed command 9. When the object is accelerated, electric current to be limited is brought into saturation by such as a driving amplifier, and when the object is decelerated, approximately two thirds of the electric current in comparison to that in acceleration is consumed so as to uniformly decelerate the object. This speed reference curve is established from the relation between a speed and a moving distance when the uniform accelerated motion is performed as will be described below. According to the relation between the speed v and the time t when the object is accelerated at uniform acceleration a, there is established such an equation as: EQU v=at (1)
In addition, according to the relation between the moving distance s and the time t when the object is accelerated at uniform acceleration a, there is established such an equation as follows: EQU s=at.sup.2 /2 (2)
Since the moving distance s to be required is an error between a target position x.sub.T and a present position x, if the time t is eliminated by such equations (1) and (2) as described above, the speed reference curve is established to be such an equation as follows: EQU v=.sqroot.2a(x.sub.T -x) (3)
In the position control system, there is performed such control as to make the position signal 15 accord with the position command 2 by such a process as that feedback compensation is applied through the compensator 12 to the position error 16 between the position command 2 and the position signal 15 detected from the object 1 by means of the position detector 14, and that the control input 13 is thereby generated.
In the positioning control apparatus of the prior art, in order to realize high-speed positioning with high accuracy with as little control input as possible, a control for permitting high-speed movement and a control for carrying out high-accurate positioning have been performed by different systems. Both control systems can be switched over by the switch 11, however, it is necessary to decide both the switching conditions and the characteristics of both control systems so that the transient response simulated in accordance with the initial position and the initial speed at the time for a switch over can be allowed. In FIG. 2, x.sub.e designates a switching position.
As may be seen from the above description, in the positioning control apparatus of the prior art, in order to perform positioning control at high speed with high accuracy, control for high-speed movement has been realized in the speed control system, and so has been done control for high-accurate positioning in the position control system, respectively, whereby control is performed by switching over both control systems. This apparatus, however, has three problems as follows:
.circle.1 Since basically two control systems are switched over, transient response will be brought about unless the speed when those systems are switched over is made completely to be null, as a result, settling time will be longer. PA1 .circle.2 Since the switching point is decided by simulating the transient response with trial and error, the control systems can not readily be designed. PA1 .circle.3 The manner of switching over the systems makes those control systems complicated as well as a lot of circuits required therein result in more cost.